A device is known from DE-OS 3,601,249 which employs a metal diaphragm in a housing which is exposed to hydraulic pressure. A strain gauge sensor on the diaphragm provides an electrical signal dependent on the deformation of the diaphragm, and hence the pressure, and the signal is amplified and used as an output. The housing is provided at one end with a socket or plug pin for coupling directly to some appliance or unit containing hydraulic pressure fluid so that the device can be left in position to monitor or measure the prevailing hydraulic pressure. The known device is difficult to manufacture and assemble and a general object of the invention is to provide an improved device.